We want to write num_frames frames to the sink, If we have more space free, we still can only write num_frames onto the sink, cause of this that check is there.

In general the old behaviour was:
Ask what is available, if not enough -> bail; ask the sink to do snd_pcm_wait for max bufferSize long; and now write num_frames data no matter if there is space or not.

That caused two times an Handle Error which flooded the log. Hopefully @manio  can link one.